This invention relates to a compressor blade for an aircraft engine whose airfoil comprises a blade core made of a fiber compound material and a metallic enclosing structure.
The blades of turbomachines, in particular of aircraft engines, are subject to considerable loads arising from centrifugal forces, gas pressure and airfoil vibrations excited by the fluid flow, but also from erosion due to ingested foreign matter or corrosion. The compressor blades, especially the fan blades, of aircraft engines are additionally loaded to a considerable extent by the impingement of birds in the event of a bird strike. The correspondingly stringent safety requirements are in opposition to the endeavor to cut the manufacturing costs and to lower the specific fuel consumption by reducing the weight of the blades and, hence, of the burst protection elements of the engine. The respective design features are similarly applicable to both, rotor blades and stator vanes.
For weight saving, blades of hollow design are known which, however, are extremely costly to manufacture and economically justifiable only in the case of large engines. Since the use of hollow-design blades in small-power engines is not effective in terms of the ratio between the achievable weight saving and the cost investment, solid titanium is being used to manufacture the correspondingly smaller blades of such engines. While these blades withstand the above-mentioned loads, they are susceptible to the vibrations caused by the fluid flow and are also heavy and costly.
Furthermore, blades in fiber compound materials have been developed which, while having a considerable weight advantage over solid blades made of an appropriate metal and possessing appropriate specific strength and high intrinsic damping characteristics against vibrations, do not satisfy the stringent strength criteria as regards erosion or a bird strike to the surface or the leading edge of the airfoil.
Specification U.S. Pat. No. 5,655,883 describes a compressor blade of a carbon fiber compound material in an epoxy resin matrix and a titanium enclosing structure whose titanium section covers the suction side as well as the leading-edge, trailing-edge and blade-tip areas.
From Specification DE 196 27 860 C1, a blade for a turbomachine is known which comprises a supporting laminated structure of fiber-reinforced plastic whose surface area is partly of fully covered with a layer of metallic fibers or filaments which are bonded to the mating fiber-reinforced plastic by means of the same resin binding agent.
Finally, Specification EP 1106783 A2 proposes a turbomachine blade whose blade-core supporting structure is made of a compound material and is surrounded by a metallic enclosing structure consisting of a plate in titanium or a similar material and a metal-felt weave welded or brazed to the inner side of the plate.
While the above-mentioned engine blades possess adequate intrinsic strength, corrosion resistance and vibration damping characteristics in combination with a significantly lower weight than the known solid metal blades, they do not satisfy the stringent safety requirements for erosion resistance and, in particular, impact strength in the event of a bird strike or an impingement of other objects. The covering structure can be damaged by prolonged erosion or a bird strike, as a result of which the blade core consisting of a fiber compound material may be destroyed. Furthermore, the known supporting and enveloping structures do not allow an aerodynamically favorable blade geometry to be produced, especially in the air inflow area.